1. Field of the Invention
The present invention relates generally to cardiac rate-responsive pacemakers, and more particularly to a cardiac rate-responsive pacemaker which responds to the average amplitude of the raw signal generated by a force transducer, such as a piezoelectric crystal or other acoustic energy transducer, used to sense physical activity associated with a user of the pacemaker.
A rate-responsive pacemaker is one wherein the basic pacing interval or rate varies as a function of some sensed parameter, such as a physiological parameter. As used herein, a broad definition of "pacing interval" or "pacing rate" is intended. This definition includes: (1) the rate at which the pacemaker provides stimulation pulses; or (2) the rate, in the case of a demand-type pacemaker, at which the pacemaker would provide stimulation pulses in the absence of naturally occurring cardiac events.
A typical rate-responsive pacemaker attempts to automatically adjust the pacing interval in order to best satisfy the physiological demands of the patient using the pacemaker. Rate-responsive pacemakers are known in the art which respond to respiration rate (see U.S. Pat. No. 3,593,718 to Krasner et al. and U.S. Pat. No. 4,567,892 to Plicchi et al.), blood temperature (see U.S. Pat. No. 4,436,090 to Cook et al.), blood oxygen saturation (see U.S. Pat. No. 4,202,339 to Wirtzfeld), or the physical activity of the patient (see U.S. Pat. No. 4,140,132 to Dahl and U.S. Pat. No. 4,428,378 to Anderson et al.).
Whatever type of physiological parameter is monitored by the rate-responsive pacemaker, some sort of sensor must be employed in order to generate an electrical signal (herein the "raw signal") which provides a measure of the selected physiological parameter. This raw signal is then used as an input signal to the circuits of the rate-responsive pacemaker in order to provide such circuits with the operating information needed to properly adjust the pacing interval.
A common problem associated with all rate responsive pacemakers relates to how the raw signal generated by the physiological sensor should be processed. That is, some sort of detection criteria must be applied in order to efficiently extract from the raw signal all proper information (e.g., true physiological activity), and exclude all improper information (e.g., noise). The prior art teaches certain signal processing techniques in order to achieve this extracting and excluding function.
For example, filtering the raw signal in order to exclude unwanted frequencies is known. Further, thresholding and frequency measurement of the signal in order to extract only those portions of the raw signal having an amplitude which exceeds a prescribed threshold level and frequency is known. Such filtering, thresholding, and frequency measurement techniques are adequate for many applications. However, for some applications these techniques of filtering, thresholding, and frequency measurement may operate to exclude relevant information and/or include irrelevant information. Accordingly, there is a need in the rate-responsive pacemaker art for improved processing circuitry for efficiently utilizing the informational content of the raw signal obtained from a physiological sensor.
Coupled with this need is the need to realize such processing circuitry inexpensively, both in terms of cost to build and power to operate, in a small space. The present invention addresses these and other objectives. Finally, the present invention must attain all of the aforesaid advantages and objectives without incurring any substantial relative disadvantage.